How to scale area light icon in viewport?
You can scale rectangle with S or when you bring cursor closer to rectangle, drag the corner handle.

But I would say you are asking how to change light Distance represented by a line. For that you would have to go to Light Properties &gt; Custom Distance and change parametr (even when it is disabled it change the widget).
It is accessible only via Eevee. Seems like it would need a touch of GUI designer :)